Chromium is widely used as an electrochemically applied coating on metal to provide wear resistance and/or reduce friction, or to affect a desired appearance. Conventionally, chromium is deposited from an electrolyte in which the chromium is in the hexavalent (Cr.sup.+6) state. Such depositions are disadvantageous in that they require expensive waste treatment procedures to reduce, if not eliminate, toxic and suspected carcinogenic waste products. Additionally, the cathode current efficiency of hexavalent chromium is experimentally found to be in a range of only 8 to 15%, depending on the type of electrolyte, because of the energy required to overcome a semi-protective cathode film before metal is deposited. At these current efficiencies, the electrochemical equivalent of chromium deposited from the hexavalent state ranges from 7.2 to 13.5 .mu.m/amps.
Chromium is rarely deposited commercially from trivalent electrolytes because most commercial processes are only capable of producing coatings of limited thicknesses, i.e. of 2.5 .mu.m or less. Such coatings are not as resistant to wear and have a different surface appearance compared with coatings produced from the widely used hexavalent chromium. Another problem in the deposition of chromium from the trivalent state is the anodic reaction which usually causes the oxidation of Cr.sup.+3 to Cr.sup.+6. This oxidation results in reduced current efficiency and increases in waste treatment costs. Undesired oxidation has been addressed in commercial trivalent chromium systems by isolating the anodic reaction chamber with appropriate barriers such as an ion selective membrane or a ceramic barrier or by using an anolyte different in composition from the bulk electrolyte. The oxidation reaction can be selected to be bromine or chlorine evolution, which occurs at a lower anodic potential than does Cr.sup.+3 oxidation. However, such solutions are disadvantageous owing to the generation of toxic halide gases.
Accordingly, a need exists for improved methods and compositions for electrodepositing chromium coatings.